The teaching-learning activity has three areas of focus: (a) subject, (b) student, and (c) style.
The author must consider all three, and establish the desired level of balance for the essay.
An expository essay has more focus on the subject matter -- it organization, impact, and applications -- than on the character of the student or the style of presentation. Wikipedia (was encyclopedia) and Wikihow (was "How-To books') and YouTube (did we have similar before?) are primarily expository essays. The goal is the transmission of the content information.
A persuasive essay has the goal of changing the student (both thoughts and actions). The focus is mostly on the life-choices of the student. So, knowing student habits, preferences, temperaments, education, ethics, morality, etc. are crucial to success. Ads and commercials and marketing brochures/flyers and vendor documentation of their products are intended to persuade you (to buy their product, change your habit/food/weight, use their computer/software/service, etc.). WyzAnt has forums, blogs, lessons, videos, etc. to persuade students and tutors to work together (with a % fee to WyzAnt). In short -- the goal is lifechange.
Style is the attention-getting aspect -- the slanted "E" in Dell, the very artistic logos, the cute (but have nothing to do with the product) ads, the bright-colored vests of safety/emergency personnel, the piped-in music in stores/offices, the unique little tune when a Windows PC starts. Effective style is unique, memorable, and multisensory (auditory/visual/sensual/...) -- you know, "cute." For either persuasive essay or expository essay, style is often "the icing on the cake."
Now, I've written a short essay here. Which type do you think it is?